As a catalyst for the polymerization of α-olefins having 3 to 10 carbon atoms to industrially produce olefin polymers, Ziegler-Natta catalysts are known. The Ziegler-Natta catalysts are in worldwide use particularly as a catalyst for the polymerization of C3, propylene, to produce polypropylene. The Ziegler-Natta catalysts can produce polypropylene having high molecular weight with high activity, and moreover can control the sequences of propylene to high degree, consequently providing polypropylene having high stereoregularity. Accordingly, isotactic polypropylene having high crystallinity and high melting point is obtained. On the other hand, the Ziegler-Natta catalysts have problems: the presence of plural kinds of active sites in the catalysts by-produces atactic polypropylene, which has irregular propylene sequence, together with the isotactic polypropylene; and in copolymerization of ethylene and an α-olefin having 3 to 10 carbon atoms, the resultant copolymers contain the α-olefin in their polymer chains in a nonuniform proportion, consequently having a wide composition distribution.
Since the discovery in 1980 by W. Kaminsky et al. of metallocene catalysts, the metallocene catalysts, too, have come to be used as a catalyst for the polymerization of α-olefins having 3 to 10 carbon atoms to industrially produce olefin polymers. The metallocene catalysts, most of which are a single metal complex, have a uniform active site, unlike the Ziegler-Natta catalysts. Thus, the metallocene catalysts have an advantage that in the copolymerization of ethylene and an α-olefin having 3 to 10 carbon atoms, they allow the resultant copolymers to contain the α-olefin in a uniform proportion and have a narrow composition distribution and have a high quality. Moreover, by converting a ligand structure of the metal complex, the C3-10 α-olefin sequence can be controlled to high degree. For example, in the polymerization of C3, propylene to produce polypropylene, the metallocene catalysts have characteristics to produce highly isotactic polypropylene as is the case with the Ziegler-Natta catalysts, but do not by-produce atactic polypropylene.
In using the metallocene catalysts for the polymerization of propylene thereby producing isotactic polypropylene, adding hydrogen into the reaction system in order to obtain a desirable molecular weight is known. It has been reported that adding hydrogen improves catalyst activity, and it has been explained that this is because a dormant catalyst species with propylene 2,1-insertion is reactivated by hydrogen (Non-Patent Literature 1). On the other hand, the more hydrogen is added, the lower the molecular weight of the resultant polypropylene becomes. Thus, especially when a high molecular weight is desired, the utility of adding hydrogen and the application scope thereof are restricted.
It has been reported that in using the metallocene catalysts to copolymerize propylene and a small amount of ethylene thereby producing an isotactic polymer (random polypropylene), increasing the amount of ethylene to be added into the reaction system improves the catalyst activity (Non-Patent Literature 2). In this case, the resultant isotactic polypropylene, by including the ethylene unit in the molecular chains, have lowered melting point. For this reason, especially when high heat resistance is desired, the addition amount of ethylene is restricted and moreover other physical properties are affected. In view of this, a method focusing on the use of ethylene in order to achieve higher activity and then using ethylene in a small amount has been proposed (Patent Literature 1): specifically, adding a small amount of ethylene (preferably about 2 wt % or more) to a propylene polymerization reaction system that used a non-bridged metallocene catalyst containing at least one kind of 2-arylindene as a ligand improved catalyst activity. In the example disclosed in this report, using bis(2-(3,5-di-t-butylphenyl)indenyl)zirconiumdichloride to produce a random polypropylene having an ethylene content of 1.9 mol % achieved improved activity and allowed the polymer to have increased molecular weight compared with where ethylene was not added, but resulted in the polymer having a melting point that was lowered from 144° C. to 137° C. Moreover, the resulting random polypropylene had tensile properties such that it showed such elastomeric properties as uniform deformation to high elongation which is followed by high recovery from the elongation. That is, when high heat resistance is desired, the problem remains that the ethylene-addition amount is restricted and moreover other physical properties are affected. On the other hand, there was a report in 1989 about a metallocene catalyst capable of producing a highly syndiotactic polypropylene that was not producible by the Ziegler-Natta catalysts (Non-Patent Literature 3): the highly syndiotactic polypropylene was produced through the polymerization of propylene utilizing a metallocene catalyst obtained by activating a Group IV transition metal compound with aluminoxane, the transition metal compound containing, as a ligand, isopropylidene(cyclopentadienyl)(9-fluorenyl) in which cyclopentadiene and fluorene are bridged by a carbon atom. The syndiotactic structure is under a “racemic” relationship in which substituents bonded to tertiary carbon (methyl groups in the case of propylene) are oriented in a direction alternating between one another, which relationship is in contrast with a “meso” relationship seen in the isotactic structure.

A polymer, when keeping an extended relationship of adjacent racemic chains, is highly syndiotactic. The above report defines the degree of the syndiotacticity using a proportion of a part having four consecutive racemic chains in the whole polymer, i.e., racemic pentad fraction. In the report, the highest racemic pentad fraction is 0.86: when this is converted to a simple racemic chain (racemic diad fraction), the racemic diad fraction is 0.96, which is significantly high. Furthermore, a polymer that is highly syndiotactic is crystalline. Since the racemic diad fraction and the racemic pentad fraction are proportional to the melting point (melting temperature) of the polymer, the polymer that has higher syndiotacticity generally shows higher melting point.
The steric influence of such adjacent substituents is as follows. While the isotactic polypropylene adopts a helical secondary structure in which three molecules of propylene are present per turn of the helix to form crystals, it has been reported that the syndiotactic polypropylene, presumably because of the alternating orientation of substituents, can adopt plural energetically-stabilized crystal structures including helical structure and planar zigzag structure depending on heat history and stress history, and that between some of these crystal structures, reversible phase transition is possible (Non-Patent Literature 4). Such difference in crystal structure greatly affects polymer material properties; it has been reported that the syndiotactic polypropylene shows characteristic thermal properties, physical properties and mechanical properties that differ from those of isotactic polypropylene. The difference in material properties caused by the difference between the syndiotactic structure and the isotactic structure is seen also in other α-olefin polymers. Thus, efficient production of polymers having a syndiotactic structure has been demanded.